Severe Accident Progression and Release Categories Analysis in Application to EPZ Evaluation for a BWR-4 NPP

After the Fukushima nuclear accident, the resetting of the boundary of the Emergency Planning Zone (EPZ) of nuclear power plants has aroused a lot of controversies in Taiwan. The regulation requires the licensees reevaluate periodically the adequacy of the EPZ boundaries based on the prevailing demographic and weather data surrounding the nuclear power plant sites. In the past, the licensees were used to adopt a simplified approach in the evaluation which is mainly derived from the results of Level-1 Probabilistic Risk Assessments (PRAs). To reflect the characteristics of the EPZ in real, especially in the wake of the Fukushima accident, the licensees decide to develop severe accident progress model to determine the plant-specific source terms for release category analysis. Besides, the regulatory body also requires that the EPZ evaluation shall consider the potential risks from other hazardous external events on the site, no longer limited to internal events as before. In this study, a Level-2 PRA is performed for Chinshan NPP, with BWR 4 reactor and Mark I containment, to define the plant damage states, potential containment failure modes, failure rates, and finally a set of the representative accident sequences for source term analysis. This study also takes into account the risk of external initiating events of public concern, i. e., seismic and tsunami, in the reevaluation of the EPZ.